When a tooth decays to the point where the inner nerves and roots become infected, dentists must oftentimes perform a root canal procedure in order to save the tooth and prevent further infection. The primary goal of a root canal is to remove all of the decayed or injured nerve while leaving the integrity of the root canal walls relatively unaffected. Preserving the integrity of the root canal is important in order to allow proper filling of the root canal such that leakage or communication between the root canal system and the surrounding and supporting tissues of the tooth is prevented.
To perform a root canal procedure, the endodontist first drills into the tooth to locate the root canal. Endodontic instruments, commonly referred to as files and reamers, are then used to clean and enlarge the root canals of the tooth. The purpose of the cleaning and enlarging procedure is to remove dead, decayed, or infected tissue from the root canals and to enlarge the root canals so that they can be filled. The reamers and files can either be finger manipulated or engine driven by the endodontist, e.g., rotary, oscillating, reciprocating, etc., and are typically of small diameter to allow them to be used in the small working environment of the root canal.
Endodontic files and reamers may employ helical flutes, as shown in U.S. Pat. No. 6,409,506 to Graybill, or non-helical flutes, as disclosed in related U.S. patent application Ser. No. 10/403,400. The build-up of debris between the flutes of the instrument and the canal walls can cause damage to the canal walls and/or lead to failure of the instrument. Therefore, during the root canal procedure it is important to evacuate the debris adequately as the file or reamer enlarges the root canal and progresses toward its apex.
One problem associated with helical fluted endodontic instruments involves the instrument self-threading or screwing into the canal. This is because helical flutes have continuous helically oriented cutting edges that dig in and thread or screw into the canal. This self-threading propensity of helically fluted instruments is a major undesirable operational characteristic that is a leading cause of instrument breakage. Many modifications of helical fluted instruments have been employed to address this drawback. Some of these modifications include varying the helical angles, varying the pitch, radial lands, and passive rake angles. In other instances, the taper of such instruments has been increased from a standard 0.02 mm:mm taper in an effort to enlarge the cross-sectional area, and increase the strength of the instrument to minimize breakage. Non-helically, or straight, fluted instruments have also been developed to address many of these issues.
Traditionally, endodontic instruments, regardless of the material of manufacture, have been manufactured by one of two fundamental methods. In one method an appropriate size blank of the desired material has a portion of its length ground into a desired cross sectional shape (i.e. square, triangular, rhomboid, etc.), taper (i.e. 0.02 mm:mm, 04 mm:mm etc.) and size. Next the ground portion of the blank is twisted by pulling the ground cross section through a set of gripping jaws while the blank is twisted at a prescribed rate. The result is the formation of an endodontic instrument with helical cutting flutes.
In the second method, an appropriate size blank of the desired material has a helical flute form of desired cross section and taper ground directly into a portion of its length. This is generally accomplished by rotating the blank of material as it passes axially one or more times past a rotating grinding wheel of proper form that is simultaneously translated perpendicular to the axis of the blank. In some variations of this method, the blank having helical flutes ground into it is pre-tapered prior to the fluting process.
In both these traditional methods, a grinding wheel of a chosen size, shape and composition running at an appropriate speed and feed with proper cutting oil type, volume, and pressure is employed to remove material from the blank to create the desired size and taper in the fluted cutting portion of the instrument. As explained in the first method, noted above, a further twisting process is commonly employed to create a helical fluted instrument when utilizing that method. However as disclosed in U.S. patent application Ser. No. 10/403,400, non-helical flute forms can also be employed successfully in endodontic instruments and are sometimes functionally superior to helical flute forms, especially in engine driven rotary applications.
In both of the methods described above, only mechanical abrasion is used to form the taper and cutting flutes of the instrument. Consequently, a substantial amount of heat is generated, and the instrument may experience mechanical stresses, burrs and other distortions. In addition, grinding wheels used for such machining must be dressed and/or replaced frequently. Another electrical discharge machining process disclosed in US 2003/0199236 A1 involves the removal and redepositing of at least a portion of the material removed on the instrument blank to form a recast layer having a second hardness of at least about 15% greater than the original hardness of the base material, thereby altering the mechanical properties of the material.
Therefore, there is a need for a method of manufacturing an endodontic instrument that removes material from an instrument blank in a rapid and precise manner, free from excessive heat, mechanical stress, burrs and other dimensional distortions, while also preserving the mechanical properties, i.e., hardness, of the original base material.